1. Field of the Invention
The present invention relates to a position location system for determining the position of a mobile communication device, and, more particularly, to a system employing two-way transmission of spread spectrum ranging signals between the mobile communication device and reference communication devices having relatively low accuracy clocks, to rapidly and accurately determine the position of the mobile communication device in the presence of severe multipath interference.
2. Description of the Related Art
The capability to rapidly and accurately determine the physical location of a mobile communication device would be of great benefit in a variety of applications. In a military context, it is desirable to know the location of military personnel and/or equipment during coordination of field operations and rescue missions, including scenarios where signals of conventional position-determining systems, such as global position system (GPS) signals, may not be available (e.g., within a building). More generally, appropriately equipped mobile communication devices could be used to track the position of personnel and resources located both indoors or outdoors, including but not limited to: police engaged in tactical operations; firefighters located near or within a burning building; medical personnel and equipment in a medical facility or en route to an emergency scene, including doctors, nurses, paramedics and ambulances; and personnel involved in search and rescue operations. An integrated position location communication device would also allow high-value items to be tracked and located, including such items as personal computers, laptop computers, portable electronic devices, luggage, briefcases, valuable inventory, and stolen automobiles. In urban environments, where conventional position determining systems have more difficulty operating, it would be desirable to reliably track fleets of commercial or industrial vehicles, including trucks, buses and rental vehicles. Tracking of people carrying a mobile communication device is also desirable in a number of contexts, including, but not limited to: children in a crowded environment such as a mall, amusement park or tourist attraction; location of personnel within a building; and location of prisoners in a detention facility.
The capability to determine the position of a mobile communication device also has application in locating the position of cellular telephones. Unlike conventional land-based/wire-connected telephones, the location of conventional cellular telephones cannot automatically be determined by emergency response systems (e.g., the 911 system in the United States) when an emergency call is placed. Thus, assistance cannot be provided if the caller is unable to speak to communicate his or her location (e.g., when the caller is unconscious, choking or detained against will). The capability to determine the position of cellular telephones could be used to pinpoint the location from which an emergency call has been made. Such information could also be used to assist in cell network management.
Naturally, in cases where a mobile communication device is being used primarily to transmit or receive voice or data information, it would be desirable to incorporate position location capabilities such that the device can communicate and establish position location at the same time without disruption of the voice or data communication.
Among convention techniques employed to determine the position of a mobile communication device is the reception at the mobile communication device of multiple timing signals respectively transmitted from multiple transmitters at different, known locations (e.g., global positioning system (GPS) satellites or ground-based transmitters). By determining the range to each transmitter from the arrival time of the timing signals, the mobile communication device can compute its position using triangulation.
The accuracy and operability of such position location techniques can be severely degraded in the presence of multipath interference caused by a signal traveling from a transmitter to the receiver along plural different paths, including a direct path and multiple, longer paths over which the signal is reflected off objects or other signal-reflective media. Unfortunately, multipath interference can be most severe in some of the very environments in which position location techniques would have their greatest usefulness, such as in urban environments and/or inside buildings, since artificial structures create opportunities for signals to be reflected, thereby causing signals to arrive at the receiver via a number of different paths.
Attempts have been made in position location systems to mitigate the effects of multipath interference. An example of a system reported to provide position location in a multipath environment is presented by Peterson et al. in xe2x80x9cSpread Spectrum Indoor Geolocation,xe2x80x9d Navigation: Journal of The Institute of Navigation, Vol. 45, No 2, Summer 1998, incorporated herein by reference in its entirety. In the system described therein (hereinafter referred to as the Peterson system), the transmitter of a mobile radio continuously transmits a modulated pseudorandom noise (PRN) sequence, with a carrier frequency of 258.5 MHz and a chipping rate of 23.5 MHz. The transmitter is battery powered and therefore can be easily transported inside a building. Four wideband antennas located on the roof of a test site receive the signal transmitted by the mobile radio. The signals are conveyed from the antennas to four corresponding receivers via low loss cable that extends from the roof to the receivers disposed in a central location. The receivers demodulate the signal transmitted by the mobile radio using an analog-to-digital (A/D) converter board disposed inside a host personal computer (PC), which samples the signal at 1.7 s intervals for 5.5 ms and processes the raw data to determine the Time of Arrival (TOA). The system uses two receiver computers, each with a dual channel A/D board inside. The output from the receiver boxes is fed into a dual channel A/D board on two host computers. Each of the host computers processes the signal on each channel of the A/D board to determine the TOA for each channel relative to a trigger common to both channels on the A/D board. The TOA algorithm is based on finding the leading edge of the cross correlation function of the PRN sequence that is available at the output of the correlator using frequency domain techniques. TOAs are transferred via wireless local area network to the RAM-drive of a third computer acting as the base computer. From the TOAs, the base computer calculates time differences (TDs) and determines the two-dimensional position of the transmitter. This position is then plotted in real time on a building overlay.
The Peterson system suffers from a number of shortcomings. The range between the target radio and each reference radio is determined by measuring the duration of time required for a signal to travel between the radios. This information can be determined from a one-way communication only if the target radio and the reference radios remain synchronized to the same time reference. That is, the transmitting radio establishes the time of transmission of the signal based on its local clock, and the receiving radio determines the time of arrival of the signal based on its local clock which must constantly be synchronized to the same time reference as the clock of the transmitter. The signal propagation duration can then be determined essentially by subtracting the time of transmission from the time of arrival.
Because the Peterson system uses this one-way measurement technique, the system requires synchronization between the clocks of the transmitter and the four receivers. Unfortunately, the precise time synchronization required to accurately measure the duration of the signal propagation cannot tolerate significant time drift of any local clocks over time. Consequently, all of the clocks of the system must be highly accurate (i.e., on the order of 0.03 parts per million (ppm)), thereby increasing the cost and complexity of the system.
The requirement in the Peterson system to keep the transmitter and receiver clocks synchronized has further implications on the accuracy of the position estimates made from the one-way ranging signals. Asynchronous events occur within each radio which cannot readily be characterized or predicted in advance. These events introduce errors in the radio with respect to knowledge of the actual time of transmission and time of arrival, thereby degrading the accuracy of the range and position estimates.
Developed to demonstrate the feasibility of indoor geolocation, Peterson""s test system does not address a number of technical issues required to construct a commercially useful system. For example, the receiver antennas are fixedly mounted (immobile) and cabled to receivers in a remote location. Consequently, the system is not adaptable to varying transmission conditions and cannot adjust to or compensate for scenarios where the radio of interest cannot communication with one or more of the reference receivers. Signal processing and analysis are performed with standard-size personal computers and other bulky experimental equipment. The system uses a relatively low chipping rate and remains susceptible to multipath interference, impacting the accuracy and operability of the system. Further, the position of radio determined by the system is only a two-dimensional position (i.e., in a horizontal plane).
Accordingly, there remains a need for a commercially viable position location system capable of quickly and accurately determining the three-dimensional indoor or outdoor position of a compact mobile communication device in the presence of severe multipath interference for use in the aforementioned practical applications.
It is an object of the present invention to rapidly, reliably and accurately determine the three-dimensional position of a mobile communication device in a variety of environments, including urban areas and inside buildings where multipath interference can be great.
It is a further object of the present invention to provide a compact, handheld or portable mobile communication device having position location capabilities useful in a wide array of applications, including location and/or tracking of people and items such as: military personnel and equipment, emergency personnel and equipment, valuable items, vehicles, mobile telephones, children and prisoners.
It is another object of the present invention to minimize the effects of interference caused by multipath signal propagation in a position location system, thereby providing highly accurate three-dimensional position estimates even under severe multipath conditions.
It is yet another object of the present invention to reduce the cost of a position detection system by avoiding the need for synchronization to the same timing reference throughout the system, thereby eliminating the need for certain expensive components, such as highly accurate clocks.
It is a still further object of the present invention to use state-of-the-art spread spectrum chipping rates and bandwidths to reduce multipath interference and improve position measurement accuracy in a position location system.
Another object of the present invention is to separate multipath interference from direct path signals to accurately determine the time of arrival of the direct path signal to accurately determine range.
Yet another object of the present invention is to minimize errors caused by processing delays that are difficult to characterize or accurately predict.
Still another object of the present invention is to provide a self-healing system, wherein a mobile communication device can adaptively rely on any combination of fixed radios and other mobile radios to determine its own position under varying communication conditions.
A further object of the present invention is to minimize design and manufacturing costs of a position-locating mobile communication device by using much of the existing hardware and software capability of a conventional mobile communication device.
A still further object of the present invention is to incorporate position location capabilities into a mobile communication device being used to transmit or receive voice or data information, such that the device can communicate and establish its position at the same time without disruption of the voice or data communication.
The aforesaid objects are achieved individually and in combination, and it is not intended that the present invention be construed as requiring two or more of the objects to be combined unless expressly required by the claims attached hereto.
In accordance with the present invention, a position location communication system provides accurate, reliable three-dimensional position location of a handheld or portable, spread spectrum communication device within milliseconds without interruption of voice or data communications. Using spread spectrum waveforms and processing techniques, the system of the present invention is capable of determining position location to an accuracy of less than one meter in a severe multipath environment.
More particularly, the system of the present invention employs a two-way, round-trip ranging message scheme in which the time of arrive of the ranging messages is accurately determined to yield accurate range estimates used to calculate the position of a mobile radio via trilateration. A master or target mobile radio transmits outbound ranging messages to plural reference radios which respond by transmitting reply ranging messages that indicate the location of the reference radio and the message turn around time (i.e., the time between reception of the outbound ranging message and transmission of the reply ranging message). Upon reception of the reply ranging message, the master radio determines the signal propagation time, and hence range, by subtracting the turn around time and internal processing delays from the elapsed time between transmission of the outbound ranging message and the time of arrival of the reply message. In this manner, the individual radios do not need to be synchronized to a common time reference, thereby eliminating the need for highly accurate system clocks required in conventional time-synchronized systems. The brief ranging messages can be interleaved with voice and data messages in a non-intrusive manner to provide position detection capabilities without disruption of voice and data communications.
To provide high accuracy range estimates, the time of arrival of the ranging messages are precisely estimated. By performing internal delay calibration, errors caused by difficult-to-predict internal transmitter and receiver delay variations can be minimized. The system uses state-of-the-art spread spectrum chipping rates and bandwidths to reduce multipath interference, taking advantage of existing hardware and software to carrying out a portion of the TOA estimation processing. Leading edge curve fitting is used to accurately locate the leading-edge of an acquisition sequence in the ranging message in order to further reduce effect of multipath interference on TOA estimates. The severity of multipath interference is determined by evaluating the pulse width of the acquisition sequence. If necessitated by severe multipath, frequency diversity is used to orthogonalize multipath interference with respect to the direct path signal, wherein an optimal carrier frequency is identified and used to estimate the TOA to minimize the impact of multipath interference.
Further, the system of the present invention is self-healing. Unlike conventional systems which require communication with a certain set of fixed-location reference radios, the system of the present invention can use a set of reference radios that includes fixed and/or mobile radios, wherein the set of radios relied upon to determine the location of a mobile communication device can vary over time depending on transmission conditions and the location of the mobile communication device. Any combination of fixed or mobile radios of known positions can be used as the reference radios for another mobile radio in the system, thereby providing adaptability under varying conditions.
The ranging and position location technique of the present invention is useful in wide variety of applications, including location and/or tracking of people and items such as: military personnel and equipment, emergency personnel and equipment, valuable items, vehicles, mobile telephones, children and prisoners.
The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of a specific embodiment thereof, particularly when taken in conjunction with the accompanying drawings wherein like reference numerals in the various figures are utilized to designate like components.